This invention relates to processes for treating polluted water. Specifically, it relates to apparatus and methods for destroying cyanide in water. The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the U.S. Air Force.
The toxic action of cyanide on living organisms by reducing or eliminating their ability to use oxygen is well known. This action at the acute toxic level is both rapid and fatal. Not only are the cyanides hazardous to animal life, but they are also toxic to aquatic plants and can interfere with normal biological processes that tend to purify streams. Fish, for example, can be killed directly by cyanide or indirectly by destruction of the organisms on which they feed or by destruction of the microorganisms responsible for normal oxygen balance in the water. In waste treatment plants that include biological processes as part of the treatment, cyanide pollutants present a problem for the same reason.
Most cyanide pollutants are currently being produced in the discharge wastes of five industrial processes: (1) metal plating; (2) case hardening of steel; (3) neutralizing of acid "pickle scum"; (4) refining of gold and silver ores; and (5) scrubbing of stack gases from gas or producer gas furnaces. The greatest source of cyanide bearing waste is the rinse water, spillages, and drippings from electroplating solutions of cadmium, copper, silver, gold, and zinc.
The conventional integrated process for destroying cyanides in such waste waters includes adding chlorine and caustic or hypochlorite to the water, allowing insoluble solid particles to settle out, and returning the water to the rinse tank. There, the chlorine and caustic immediately react with the metal-cyanide compound on the plated part or in the water to produce harmless precipitates and the gases, carbon dioxide and nitrogen, the escape into the atmosphere.
Although this process is effective, it is somewhat hazardous and expensive. Chlorine, itself, is very toxic, corrosive, and difficult to handle, as are the caustic and hypochlorite. Special equipment and controls are required for storage and for addition of these materials to the water. Also, in a closed system wherein the rinse water is repeatedly recycled, salt buildup in the water necessitates periodic dumping and addition of fresh water.